1. Field of the Invention
This invention relates to a method for treating the surface of crystalline silicon (c-Si) and amorphous silicon (a-Si) and more particularly to such a method which provides improved effect in impurity removal processing and surface protection processing by adhesion of phosphorus, boron, arsenic or the like.
2. Prior Art Statement
Cleaning techniques are coming to play an increasingly important role in the production of high-density, high-performance semiconductor elements. This is particularly true of crystalline silicon and amorphous silicon for use as device substrates since the cleanness of the surface of these materials can greatly affect the product yield and performance. It has therefore long been the practice to take utmost care in this respect.
However, even crystalline silicon substrates and amorphous silicon substrates that have been transported through clean air and been washed with pure water soon suffer chemical bonding to their surfaces of a natural oxide film layer or of carbon compounds, sodium and other impurities to a thickness of several atoms.
Because of this, it has been the practice in the case of crystalline silicon substrates, for instance, to remove mainly such impurities as oxygen and carbon from the substrate surface by heating the substrate to over 900.degree. C. in an ultra-high vacuum exceeding 10.sup.-8 Torr, thereby sublimating these impurities along with the oxide film.
While this high-vacuum processing is suitable as a pretreatment for molecular beam epitaxy (MBE), it is not adaptable for use with chemical vapor deposition (CVD) in which a gas source is used. Therefore, as a pretreatment for CVD, it has been the practice to remove the surface to an appropriate thickness by an etching removal process in which a gas such as of chlorine (Cl.sub.2) or hydrochloric acid (HCl) is passed over the surface of the crystalline silicon held at a high temperature, generally in the vicinity of 900.degree. C.
When such a sublimation method is carried out in a high-temperature, ultra-high vacuum environment, however, impurities released from the alumina or boron nitride (BN) support for the heater for heating the crystalline silicon being treated diffuse into the substrate or are included in the growth layer and have an adverse effect on the electrical properties. For example, in the case of a Schottky junction, ohmic contact or the like, it may be impossible to obtain the desired properties of the junction formed at the interface between the substrate and the growth layer, or, generally, a large number of lattice defects occur in the growth layer, making it impossible to grow a good quality crystal.
On the other hand, as an improvement in the etching removal technique, there has recently been developed a method in which surface treatment is carried out with a mixed gas of hydrogen and chlorine under reduced pressure at a relatively low temperature between 500.degree. and 600.degree. C. In fact, however, this improved method has difficulty severing the strong bond between silicon and oxygen and further entails a problem regarding maintenance of the equipment used since the chlorine and hydrogen chloride gases tends to corrode the piping and metal chamber of the equipment.
These two conventional methods are, moreover, limited in application to crystalline silicon type materials and are totally unusable with amorphous silicon type materials because of their excessively high treatment temperatures.